In-situ continuous coke deposit removal by catalytic steam gasification

ABSTRACT

A coke removal system removes coke deposits from the walls of a high temperature passage in which hydrocarbon fuel is present. The system includes a carbon-steam gasification catalyst and a water source. The carbon-steam gasification catalyst is applied to the walls of the high temperature passage. The water reacts with the coke deposits on the walls of the high temperature passage to remove the coke deposits from the walls of the high temperature passage by carbon-steam gasification in the presence of the carbon-steam gasification catalyst.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a divisional application of U.S. patent application Ser. No.11/431,810 filed on May 10, 2006, now U.S. Pat. No. 7,513,260

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of removing cokefrom high temperature hydrocarbon systems.

Thermal management is a significant challenge in advanced aircraft,rocket, and missile engines. Increasing aircraft speed and enginethrust-to-weight ratio results in large, simultaneous increases in heatload and temperature of the air available for cooling. Engine andvehicle cooling must therefore be accomplished by using the fuel as acooling means. One method of cooling advanced engines is the applicationof high heat sink fuel cooling technology. Although cryogenic fuels,such as liquid methane and liquid hydrogen, can provide sufficientcooling, they present issues in the areas of cost, logistics,operations, and safety. By contrast, conventional liquid hydrocarbonfuels undergoing endothermic chemical cracking, catalytically and/orthermally, offer the required cooling capacity without the problemsassociated with cryogenic fuels. The primary products of the endothermicreaction are gaseous fuels with short ignition delay times and rapidburning rates. In addition, waste heat absorbed by the fuel can bereturned to the system, enhancing performance and system efficiency.

However, the decomposition of hydrocarbon fuel at elevated temperaturesleads to coke deposition within the fuel passages. Coke typicallyconsists of approximately 80% to 95% carbon by weight with the balancecomprising sulfur, nitrogen, inorganic materials, ash, and small amountsof oxygen. The coke deposits which form in the heat exchangers,reactors, and on the fuel system component walls degrade heat transferand fuel flow characteristics and, if left unchecked, can lead to systemfailure. The extent to which the benefits of high endothermichydrocarbon fuel cooling technology can be realized is thus directlyrelated to the ability to mitigate against coke formation.

BRIEF SUMMARY OF THE INVENTION

A system removes coke deposits from the walls of a high temperaturepassage in which hydrocarbon fuel is present. The system includes acarbon-steam gasification catalyst and a water source. The carbon-steamgasification catalyst is applied to the walls of the high temperaturepassage. The steam from the water source reacts with the coke depositson the walls of the high temperature passage to remove the coke depositsfrom the walls of the high temperature passage by carbon-steamgasification in the presence of the carbon-steam gasification catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a high temperature system using high heatsink fuel cooling technology.

FIG. 2 is a schematic view of an intermediate portion of the hightemperature system.

FIG. 3 is a diagram of a method of removing coke deposits from the wallsof the high temperature system.

FIG. 4 is a partial cross-sectional view of walls of the hightemperature system with coke deposits.

FIG. 5 is a graph showing rate of coke deposition on a wall of the hightemperature system as a function of temperature.

FIG. 6 is an enlarged schematic view of a coke deposit on the wall ofthe high temperature system and the steam gasification reaction at thewall of the high temperature system for removing the coke deposit.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of high temperature system 10 using highheat sink fuel cooling technology with catalytic carbon-steamgasification coke removal. High temperature system 10 can be any systemrequiring high operating temperatures, such as a gas turbine or ahypersonic scramjet. High temperature system 10 generally includes fuelreservoir 12, water or steam supply system 14, heat exchanger 16,injector 18, combuster 20, and piping 22. Hydrocarbon fuel is stored infuel reservoir 12 and is pumped to heat exchanger 16 through piping 22when needed. After the hydrocarbon fuel has been reacted, it is passedthrough injector 18 to combuster 20. Combuster 20 provides power, orpropulsion, to high temperature system 10.

Hydrocarbon fuel flows through high temperature system 10 and can be anytype of hydrocarbon, including any hydrocarbon fuel that is susceptibleto coking at elevated temperatures such as gas turbine fuels and otherkerosene-type hydrocarbon fuels. For example, in space and rocketapplications, the hydrocarbon fuels discussed above include methane. Athigh temperatures, the hydrocarbon fuel flowing through heat exchanger16 can crack, depositing coke within the walls of heat exchanger 16 anddownstream of heat exchanger 16. High temperature system 10 removes cokedeposits through catalytic carbon-steam gasification. Catalyticcarbon-steam gasification provides a cost-effective and simple solutionfor removing coke deposits from high temperature system 10, therebyincreasing hydrocarbon fuel cooling capacity. This in turn extends thelife and/or runtime of high temperature system 10. The addition ofwater/steam to the hydrocarbon fuel flowing through heat exchanger 16also enhances combustion efficiency, lowers the lowest-minimum fueltemperature for operability, and reduces emissions.

Heat exchanger 16 of high temperature system 10 operates at temperaturesof at least 700 degrees Fahrenheit (° F.), and preferably attemperatures of at least 900° F. At these temperatures, thecarbon-carbon bonds of the hydrocarbon fuel break, absorbing heat. Thehydrocarbon fuel passing through high temperature system 10 is used tocool the heat transfer medium flowing through heat exchanger 16, such ascompressed, high temperature air, or to cool a structure, such as thewalls of combuster 20 or high temperature surfaces of a vehicle. Thus,the hydrocarbon fuel is used as a heat sink or a cooling source to meetthe cooling requirements of high temperature system 10.

FIG. 2 shows a more detailed schematic view of an intermediate portion24 of high temperature system 10 (shown in FIG. 1). Intermediate portion24 of high temperature system 10 generally includes fuel reservoir 12,water supply system 14, heat exchanger 16, computer control system 26,hydrocarbon fuel flow meter 28, hydrocarbon fuel flow controller 30, andDC power supply 32. As previously mentioned, hydrocarbon fuel is pumpedfrom fuel reservoir 12 to heat exchanger 16 when needed. Flow meter 28is located downstream of fuel reservoir 12 and measures the flow rate ofhydrocarbon fuel flowing from fuel reservoir 12. The measurement is sentthrough hydrocarbon flow signal line 34 to computer control system 26where it is compared to a predetermined target flow rate. Computercontrol system 26 then sends a signal to flow controller 30 throughhydrocarbon flow control line 36 to adjust the flow rate of hydrocarbonfuel from fuel reservoir 12 to the desired flow rate. In one embodiment,flow controller 30 is a flow valve.

Direct current is supplied to heat exchanger 16 from DC power supply 32through power line 38, which is controlled by computer control system26. As the hydrocarbon fuel is passed through heat exchanger 16, itundergoes the desired reaction. For example, the carbon-carbon bonds ofthe hydrocarbon fuel are broken to produce smaller molecules that willmore readily burn in combuster 20 (shown in FIG. 1). The volume ofhydrocarbon fuel in heat exchanger 16 and the wall surface temperaturesof heat exchanger 16 can be measured by performing an energy balance,determined by the energy balance equation:Q _(heat sink) =Q _(input) −Q _(loss)where Q_(heat sink) is the heat sink of the hydrocarbon fuel that flowsthrough heat exchanger 16, Q_(input) is the energy put into heatexchanger 16 by DC power supply 32, and Q_(loss) is the energy lost tothe surroundings.

Water/steam supply system 14 generally includes water reservoir 40,water flow meter 42, water flow controller 44, and piping 46. Water isstored in water reservoir 40 and is transported through piping 46 topiping 22 of high temperature system 10 upstream of heat exchanger 16.The water is combined with the hydrocarbon fuel flowing through piping22 of high temperature system 10 upstream of heat exchanger 16. Similarto high temperature system 10, the flow rate of the water flowing fromwater reservoir 40 is measured by water flow meter 42, which sends asignal back to computer control system 26 via water flow signal line 48.Computer controller system 26 then sends a signal through water flowcontrol line 50 to water flow controller 44. The flow rate of the waterflowing to heat exchanger 16 is adjusted by flow controller 44 dependingon the needs of heat exchanger 16. The hydrocarbon fuel and water arethus combined and introduced into heat exchanger 16 simultaneously asone fluid. In one embodiment, the water constitutes approximately 10% ofthe total weight of the hydrocarbon fuel and water combined. The waterpreferably constitutes approximately 5% of the total weight of thehydrocarbon fuel and water combined, a more preferably constitutesapproximately 2% of the total weight of the hydrocarbon fuel and watercombined, and most preferably constitutes approximately 1% of the totalweight of the hydrocarbon fuel and water combined.

FIG. 3 shows a diagram of a method of removing coke deposits from thewalls of the heat exchanger 16. A catalyst is first coated on the wallsof heat exchanger 16, Box 52. Hydrocarbon fuel is passed through heatexchanger 16 and the flow rate of the fuel is measured and adjusted asnecessary, Box 54. As shown in Box 54, water is introduced into thehydrocarbon fuel upstream of heat exchanger 16. The flow rate of thesteam is also measured such that it can be adjusted as necessary toremove the coke deposits from the walls of heat exchanger 16 withoutover-diluting the hydrocarbon fuel. As the hydrocarbon fuel and thewater pass through heat exchanger 16, the steam reacts with the cokedeposits on the walls of heat exchanger 16, converting the coke andsteam to hydrogen and carbon monoxide to be used as fuel downstream incombuster 20. The coke deposits are thus removed from the walls of heatexchanger 16 through catalytic carbon-steam gasification, Box 56.Although FIG. 3 describes removing coke deposits from heat exchanger 16,coke deposits can also be removed from other high temperature passagesof high temperature system 10 where coke may deposit, such as a fuelnozzle or a fuel valve.

FIG. 4 shows an enlarged, partial cross-sectional view of heat exchanger16. At high operating temperatures, hydrocarbon fuel is not stable anddeposits coke 58, or carbon-rich deposits, on wall surfaces 60 of heatexchanger 16 through which the hydrocarbon fuel passes. As hydrocarbonfuel flows through heat exchanger 16, coke deposits 58 continue to buildon wall surfaces 60 of heat exchanger 16. If left unchecked, cokedeposits 58 can cause damage and lead to failure of high temperaturesystem 10 (shown in FIG. 1). To prevent failure of high temperaturesystem 10, coke deposits 58 must be removed from high temperaturepassages of high temperature system 10.

FIG. 5 is a graph showing the rate of coke deposition on the walls of ahigh temperature system as a function of temperature. Coke depositionrates depend on bulk fuel temperatures as well as wall surfacetemperatures. As can be seen in FIG. 5, at high temperatures, cokedeposition rates increase exponentially with increasing temperature. Thehighest coke deposition rates will therefore occur at surfaces where thetemperature is highest. A benefit of using catalytic carbon-steamgasification to remove coke deposits on wall surfaces is that thereaction rate of catalytic carbon-steam gasification increases as thetemperature of the wall surface increases. Thus, as the temperatureinside high temperature system 10 increases and the rate of cokedeposition increases, the rate of coke removal by catalytic carbon-steamgasification will also increase.

FIG. 6 shows an enlarged schematic view of a coke deposit 58 on a wallsurface 60 of heat exchanger 16 and the chemical reaction at wallsurface 60 during catalytic carbon-steam gasification. Prior to passingthe hydrocarbon fuel through heat exchanger 16, a catalyst 62 is coatedon wall surfaces 60 of heat exchanger 16. Catalyst 62 acts to catalizethe reaction of coke with the steam introduced by water supply system 14(shown in FIG. 1). Examples of steam gasification catalysts include, butare not limited to: alkaline metal salts and alkali earth metal salts.Examples of alkaline metal salts include the IA group of the periodictable of elements, such as Na₂CO₃, K₂CO₃, Cs₂CO₃. Examples of alkaliearth metal salts include the IIA group of the periodic table ofelements, such as MgCO₃, CaCO₃, SrCO₃, BaCO₃.

Coke deposits 58 are removed from wall surfaces 60 coated with catalyst62 by adding small amounts of water to the hydrocarbon fuel stream. Thewater is added to the hydrocarbon fuel before the hydrocarbon fuelpasses through heat exchanger 16. As the fuel and steam pass throughhigh temperature system 10, coke deposits 58 react with the steamthrough catalytic carbon-steam gasification, producing hydrogen andcarbon monoxide:C(coke)+H₂O→H₂+CO

Catalytic steam gasification of coke is a very strong endothermicreaction, increasing the overall heat sink capability of the hydrocarbonfuel. The endothermic reaction absorbs heat in heat exchanger 16 to coolthe heat transfer medium flowing through heat exchanger 16 or to cool astructure, preventing the temperature of high temperature system 10 fromreaching damaging levels. Additionally, the products of the catalyticcarbon-steam gasification reaction can also be used as fuel. Carbonmonoxide and hydrogen gas have short ignition times and are easilyburned in combuster 20. Thus, using the products of the reaction as fuelincreases the efficiency of high temperature system 10.

The system of the present invention removes coke deposits from hightemperature systems using a catalyst, hydrocarbon fuel, and water. Thecatalyst is coated on the walls of the high temperature passage prior topassing hydrocarbon fuel through the high temperature passage. Water isintroduced into the hydrocarbon fuel stream upstream of the hightemperature passage and is introduced into the high temperature passagesimultaneously with the hydrocarbon fuel. The steam reacts with the cokedeposits through catalytic steam gasification to produce hydrogen andcarbon monoxide, removing the coke deposits from the walls of the hightemperature chamber. The carbon monoxide and hydrogen are subsequentlyused as fuel when the hydrocarbon fuel is combusted. The endothermiccatalytic carbon-steam gasification reaction also serves to cool thehigh temperature passage, using the hydrocarbon fuel as a heat sink.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A high heat sink fuel cooling system comprising: a combustor forproviding propulsion to the high heat sink fuel cooling system; a heatexchanger for cooling walls of the combustor; a carbon-steamgasification catalyst in direct contact with walls of the heatexchanger; a hydrocarbon fuel-water supply connected to the heatexchanger to provide a hydrocarbon fuel-water stream that passes throughthe heat exchanger, reacts with the carbon-steam gasification catalyston the walls of the heat exchanger to remove coke deposits by catalyticcarbon-steam gasification and provides a heat sink for the heatexchanger; and an injector at a downstream end of the heat exchanger forreceiving the hydrocarbon fuel-water stream from the heat exchanger andpassing it to the combustor.
 2. The system of claim 1, wherein thecarbon-steam gasification catalyst is selected from the group consistingof: alkaline metal salts, alkali earth metal salts, and combinationsthereof.
 3. The system of claim 2, wherein the carbon-steam gasificationcatalyst is selected from the group consisting of Na₂CO₃, K₂CO₃, Cs₂CO₃,MgCO₃, CaCO₃, SrCO₃, and BaCO₃.
 4. The system of claim 3, wherein thecarbon-steam gasification catalyst is a cesium (Cs) salt.
 5. The systemof claim 1, wherein the heat exchanger operates at a temperature of atleast 700 degrees Fahrenheit.
 6. The system of claim 5, wherein the heatexchanger operates at a temperature of at least 900 degrees Fahrenheit.7. The system of claim 1, wherein the hydrocarbon fuel-water streamconstitutes between approximately 1 percent and approximately 10 percentwater by weight.
 8. The system of claim 7, wherein the hydrocarbonfuel-water stream constitutes between approximately 1 percent andapproximately 5 percent water by weight.
 9. The system of claim 8,wherein the hydrocarbon fuel-water stream constitutes betweenapproximately 1 percent and approximately 2 percent water by weight. 10.The system of claim 1, wherein the hydrocarbon fuel-water stream passesfrom the heat exchanger through the injector into the combuster andhydrocarbon fuel is burned.
 11. A system comprising: a hydrocarbon fuelreservoir; a water source; a heat exchanger having a catalyst in directcontact with walls of a passage for reacting with the hydrocarbon fuel;a delivery system connected to the hydrocarbon fuel reservoir and thewater source for delivering a hydrocarbon fuel-water stream that passesthrough the heat exchanger and reacts with the catalyst on the walls ofthe passage to remove the coke deposits by catalytic steam gasification;and a combuster, wherein hydrocarbon fuel, carbon monoxide, and hydrogengas are burned after the hydrocarbon fuel-water stream passes throughthe heat exchanger and a wall of the combustor is cooled by the heatexchanger.
 12. The system of claim 11, wherein the catalyst is selectedfrom the group consisting of: alkaline metal salts, alkali earth metalsalts, and combinations thereof.
 13. The system of claim 12, wherein thecatalyst is selected from the group consisting of Na₂CO₃, K₂CO₃, Cs₂CO₃,MgCO₃, CaCO₃, SrCO₃, and BaCO₃.
 14. The system of claim 11, wherein thehydrocarbon fuel-water stream constitutes between approximately 1percent and approximately 10 percent water by weight.
 15. The system ofclaim 14, wherein the hydrocarbon fuel-water stream constitutes betweenapproximately 1 percent and approximately 5 percent water by weight. 16.The system of claim 15, wherein the hydrocarbon fuel-water streamconstitutes between approximately 1 percent and approximately 2 percentwater by weight.
 17. The system of claim 11, wherein the heat exchangeroperates at a temperature of at least 700 degrees Fahrenheit.
 18. Thesystem of claim 17, wherein the heat exchanger operates at a temperatureof at least 900 degrees Fahrenheit.
 19. The system of claim 11, furthercomprising: an injector at a downstream end of the heat exchangerthrough which the hydrocarbon fuel-water stream passes before enteringthe combuster.